A power circuit-breaker with remote-controllable operation threshold has already been proposed in the German published patent application No. 2.404.234.
Apart from the embodiment comprising two limiting elements in series, such as bimetallic strips, one of which can be shunted by the remote-control member, this patent also suggests a circuit-breaker with a single thermal limiting member such as a bimetallic strip with an additional heating device of said bimetallic strip which is set under tension by the remote-control member for lowering the operation threshold of the release device by creating a predeformation of the bimetallic strip.
The embodiment with two limiting elements in series is complex and cumbersome, making it impossible to house the device in casings of usual size. Moreover, when two bimetallic strips as proposed in said patent are concerned, there is a thermal influence between the bimetallic strips which makes practically impossible the adjustment of the two thresholds. This disadvantage is still more marked where, in the case proposed as only solution in said patent for an embodiment with a single detector element, the single bimetallic strip is combined with a heating device which is switched on by the centralized remote control receiver for reducing the intensity at which the circuit-breaker cuts out. In addition to the fact that the heating device consumes continuously during the load-shedding period, the influence of an additional heating device on the switching out threshold of the bimetallic release device is very indetermined and the adjustment of the switching out threshold is practically impossible.
A power circuit-breaker with remote-controllable operation threshold is disclosed in French Pat. No. 79.21897 dated Aug. 31, 1979, wherein the remote control is provided by the switching on or off or modification of the value of a derivation resistor mounted in parallel with a detector of the release so as to modify the fraction of the total intensity acting on said release detector. This embodiment has the advantage that the operation threshold of the release detector corresponds, whatever the adjustment of the threshold, to the same intensity flowing through the release detector, thereby facilitating the adjustment and providing stability of the operation. The contactor of the remote control providing the switching on or off of the derivation resistors is not subjected to a voltage at the terminals during the cut out, therefore subject to the formation of an arc, since it is shunted at least by the release detector, but it must nevertheless have a very low resistance for letting through a high intensity, which can be the source of problems for its manufacture and its size.